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Re: [AtkinBoats] Re: Tunnel hull question

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  • Ronald Fossum
    I m going to build a version of Rescue Minor, and to that end have done a LOT of research - both into the Sea Bright Skiff and the Atkin tunnel stern used on
    Message 1 of 2 , Jun 11, 2005
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      I'm going to build a version of Rescue Minor, and to that end have done a LOT of research - both into the Sea Bright Skiff and the Atkin tunnel stern used on them. Yes, William Atkin developed something that is quite unique, both in design and performance.

      I compared the various Atkin tunnel sterns used on the Sea Bright Skiff. From the 17' Heron to the (almost) 29' Nanuk III there is very little difference in the ratio of the length of the tunnel to the waterline length in ALL the designs, and only about 3" difference in the location of the propeller as measured from the transom. According to normal tunnel writings, the rounded tunnel - by keeping the tips of the propeller blades closer to the hull (1" recommended) - should be more efficient but in fact the "inverted V" shape used in Rescure Minor and other hard chine hulls gives better performance. I think two things come into play here:
      1. The hard chine gives a better overall "flat" shape aft (straighter buttuck lines which give higher speeds);
      2. The hard chine construction can be lighter in weight - and lighter weight equals more speed.
      This is particularly true with Rescue Minor. Although there are no figures are given for the weight of the various Sea Bright tunnel stern designs, I fed Rescue Minor into the Carlson Hulls software and came up with a figure under 1,000 lbs. total.

      I've also read (and observed) that hard chine V bottom boats whose chines do not "quite" touch the water require less horsepower to achieve a given speed. Rescue Minor is the only one of Billy's Sea Bright tunnel stern designs to do this. I think, in part, that weight once more has a great deal to do with this, but the wetted surface area vs. displacement is also a part.

      Robb White has a website with his "Rescue Minor" on it at: http://www.robbwhite.com/rescue.minor.html Take a look, there's a lot of good info there. He's also contributed to a number of threads on this forum. Search with "rescue minor" and/or "robb white". He mentions the (belt drive) reduction ratio from his tractor engine to the prop shaft. He also uses a different dia. and pitch for the propeller (smaller dia., less pitch, higher rpms) than Atkin recommends. I think that if Robb were to use the prop recommended by Atkin and reworked his reduction ration, he'd get even better performance - but he's happy with what he's got - and that's the bottom line.

      I really don't know if any of the present boat designers could get much of anything from the Atkin tunnel stern designs. Maybe Dave Gerr (or Phil Bolger if he could be interested in it). It isn't a matter of competence, it's a matter of "art" and "feel" for the form. Shame Billy isn't still alive. I'd love to pick his brain on this one!

      Ron Fossum

      ----- Original Message -----
      From: Sal's Dad
      To: AtkinBoats@yahoogroups.com
      Sent: Saturday, June 11, 2005 3:49 AM
      Subject: [AtkinBoats] Re: Tunnel hull question


      > From: James Greene
      > Subject: Tunnel hull question ...
      >
      > I'm thinking of building a tunnel hull boat and I know that Atkin boats
      > (some of them anyways) are apparently very technologically advanced in
      > terms of thelr tunnel hull designs. I'm hoping to build the most
      > advanced design they offer ...
      >
      > Does Rescue Minor have the most technologically advanced tunnel hull
      > design?

      I, too am getting serious about building a tunnel hull. My understanding,
      though far from complete, is that Rescue Minor was designed in the 40's, and
      Shoals Runner in the 50's. The designs are similar, but with significant
      differences in hull shape. Presumably there are other designs of this style
      and size, but they are not in the catalogue.

      Robb White is the reigning expert on the small tunnel hulls, and has
      reported extensively on his "Rescue Minor" in MAIB. It should be noted that
      RM is the NAME of his boat; he did NOT build to the plans. His has a
      rounded underbody rather than hard-chined, and made other functional
      changes, in addition to some very nice above-the-waterline 'improvements'.
      Robb did his own design, engineering, and mechanical work (in addition to
      his remarkable wooden boat-building).

      The only other Tunnel Stern I am aware of being built is a River Belle,
      built by my friend Alex Hadden for a client, a few years ago. I spoke with
      Alex yesterday, and got performance data on that boat, "Noble Cab", a real
      gold-plater yacht.

      Dave Gerr, the Naval Architect and author, has designed a number of much
      larger tunnel-stern yachts, but they are effectively displacement vessels -
      http://www.gerrmarine.com/ . Gerr did the engineering on Noble Cab's drive
      train. He predicted "at best" 11 knots with the 100 hp Yanmar and
      appropriate reduction/prop, in sea trials she did 15 knots, burning 1 gph.

      When I get a few minutes I will ask Robb white for a summary of "RM"s
      machinery and performance - It's an 18 hp Kubota, but I don't recall details
      of gearing, prop, displacement, or speed data. It would be nice to assemble
      data on all the known tunnel-sterns, and then ask Gerr, White, and others
      for help in understanding the possibilities and limits of the form.

      Can anybody else provide information, resources, or advice? Are there any
      other tunnel-hulls out there?

      Sal's Dad (Curtis Betts)



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